When a semiconductor layer with high composition ratio of In is used for a channel layer of a field effect transistor, since it has high electron mobility and high electron concentration, it provides excellent high frequency characteristics. For example, a technique for increasing the composition ratio of In in the channel layer of a field effect transistor on an InP substrate from 0.53 to 0.65 and thereby improving electron mobility and transconductance is known. This technique is disclosed in Applied Physics Letters, Vol. 52, No. 9, 1988, pp. 728-730. However, even if the composition ratio of In is increased, as is disclosed in Journal of Applied Physics, Vol. 62, 1987, pp. 2880-2884, if x.gtoreq.0.7, the electron mobility of In.sub.x Ga.sub.1-x As was deteriorated and more higher characteristics could not be expected.
As a conventional method for preventing electron mobility from deteriorating, a structure for disposing a layer with a thickness of 4 nm that contains In whose composition ratio is equal to lattice constant of a substrate in a hetero interface of a field effect transistor having a semiconductor layer with high composition ratio of In was proposed in "Technical Report of IEICE ED92 -102,CPM92-139" The Institute of Electronics, Information and Communication Engineers, Japan, 1992. FIG. 5 shows the structure of the field effect transistor.
As shown in FIG. 5, an In.sub.0.52 Al.sub.0.48 As layer 10 is disposed on a high resistance InP substrate 9. An In.sub.0.8 Ga.sub.0.2 As layer 11 with a large composition ratio of In is disposed on the In.sub.0.52 Al.sub.0.48 As layer 10. An In.sub.0.53 Ga.sub.0.47 As layer 12 of whose is as thick as the lattice constant of the base substrate 9, and is low composition ratio of In, is disposed on the In.sub.0.8 Ga.sub.0.2 As layer 11. A non-doped In.sub.0.52 Al.sub.0.48 As layer 13 is disposed on the In.sub.0.53 Ga.sub.0.47 As layer 12. An n-type In.sub.0.52 Al.sub.0.48 As layer 14 is disposed on the non-doped In.sub.0.52 Al.sub.0.48 As layer 13. The electron mobility of the structure with the In.sub.0.53 Ga.sub.0.47 As layer 12 shown in FIG. 5 was more improved by 9% than that of the structure without the In.sub.0.53 Ga.sub.0.47 As layer 12.
However, in the conventional field effect transistor shown in FIG. 5, the thickness of the In.sub.0.53 Ga.sub.0.47 As layer 12, which is the second semiconductor layer, is as thick as 4 nm. Thereby the ratio of electrons that exist in the In.sub.0.8 Ga.sub.0.2 As layer 11 as the first semiconductor layer is 85% of a total electron of the first semiconductor layer and the second semiconductor layer (as shown in FIG. 3).
Thus, since electrons flow in both the In.sub.0.8 Ga.sub.0.2 As layer 11 and the In.sub.0.53 Ga.sub.0.47 As layer 12, the characteristics of the In.sub.0.8 Ga.sub.0.2 As layer 11 with high electron mobility and high electron saturation velocity could not be satisfactorily obtained as the characteristics of the field effect transistor.